Generally, a nuclear plant includes hundreds of systems having separate functions. These systems are roughly classified into a nuclear steam supply system (NSSS) having a nuclear reactor as its main element, a turbine/generator system having a generator driven by supplied steam and other auxiliary equipments. Here, the nuclear reactor controls large amount of energy instantaneously emitted during a nuclear reaction to be slowly released so that nuclear energy can be utilized for generating electricity.
However, when molten core material discharged from a damaged reactor vessel during a severe accident in the nuclear plant is not effectively retained and cooled, the discharged molten core material may ablate the concrete structure of a reactor cavity, contaminate neighboring soil, and threaten the structural integrity of a containment building.
Accordingly, apparatuses and methods for retaining and suitably cooling the discharged molten core material have been constantly studied.
FIG. 1 illustrates a conventional apparatus for retaining and cooling discharged molten core material, which is disclosed in a Korean Patent No. 0265320.
In the conventional apparatus shown in FIG. 1, a multi-layered cell-type cooling water retention tank 110 is installed below a reactor vessel 100 and filled with cooling water 120 in order to sequentially cool ultrahigh-temperature molten core material discharged from the reactor vessel 100.
However, the so-constructed conventional apparatus cannot be installed in a containment building for existing nuclear reactors because the containment building must be greatly altered in its compartment structure so as to install the apparatus therein.
Also, the apparatus has a problem in that the ultrahigh-temperature molten core material melts the cooling water retention tank 110 and then reacts directly with the cooling water in the cooling water retention tank 110 thereby to generate steam violently, whereby a steam explosion may undesirably occur.
As another conventional apparatus for retaining and cooling the discharged molten core material, FIG. 2 illustrates a core catcher having elongated molten core material catch parts, disclosed in Japanese Patent Publication No. 1996-43575.
As shown in FIG. 2, the conventional apparatus has a plurality of molten core material catcher parts 130 installed below a reactor vessel 100. In the apparatus, molten core material discharged from the reactor vessel 100 during a severe accident in a nuclear plant is retained in the molten core material catcher parts 130, and the retained molten core material is cooled by cooling water 120 filled around the molten core material catcher parts 130.
However, the above-described apparatus cools the molten core material of high temperature by bringing the molten core material into indirect contact with the cooling water. Accordingly, heat transfer efficiency between the molten core material and the cooling water is low and thus a very large cooling water tank is required.
Also, there has been proposed an apparatus for actively cooling the molten core material by using a pump. However, the pump may not be operated when a severe accident occurs in a nuclear plant, whereby the apparatus has low reliability. Further, the cooling ability of the pump is degraded as time goes on, whereby the decay heat of the molten core material cannot be effectively removed.
Accordingly, there is required an improved apparatus for passively cooling and retaining the molten core material, which can solve the above-described problems of the related art.